MG12 - Talk detail
 

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Talk

Abstract

The fireshell equations of motion

The Fireshell originating a Gamma-Ray Burst (GRB) encompasses an optically thick regime followed by an optically thin one. In the first one the fireshell self-accelerates from a Lorentz gamma factor equal to 1 all the way to 200-300. The physics of this system is based on the continuous annihilation of electron-positron pairs in an optically thick e+e- plasma with a small baryon loading. In the following regime, the optically thin fireshell, composed by the baryons left over after the transparency point, ballistically expands into the CircumBurst Medium (CBM). The kinematics and dynamics of the fireshell during both regimes will be analyzed, and a comparison with corresponding treatments in the current literature will be presented. Particular attention will be devoted to the analysis of the transparency point, since it represents both the final outcome of the optically thick regime and the initial condition of the optically thin one. Some consequences on the interpretation of the observed GRBs' light curves and spectra will be presented. In particular, it will be presented the possible identification of the proper GRB (P-GRB), the flash emitted when the fireshell becomes transparent, in the case of GRB 080916C, whose full details will be presented in a next talk by L. Izzo et al. In the present talk, in the optically thick regime the fireshell will be approximated, for simplicity, as an expanding shell with constant width in the laboratory frame, following Ruffini et al., A&A, 350, 334 (1999). Possible deviations from such an approximation will be discussed in a next talk by G. De Barros et al.

The extended afterglow luminosity evolution over the equitemporal surfaces

Due to the ultrarelativistic velocity of the fireshell (Lorentz gamma factor 10^2 - 10^3), photons emitted at the same time in the laboratory frame (i.e. the one in which the center of the fireshell is at rest) from the fireshell surface but at different angles from the line of sight do not reach the observer at the same arrival time. Then, the signal we detect at a given value of the arrival time is a superposition of signals coming from different times in the laboratory frame. Therefore, they correspond to different values of the fireshell radius. Within the fireshell model, we trace back the beginning of the "plateau" phase in the GRB afterglow X-ray light curves to a collision between the decelerating front layer of the fireshell and a slower inner shell. It occurs at the end of the prompt emission phase. To determine the radius at which this collision occurs, and therefore the dynamics of the slower inner shell, it is fundamental to analyze the distribution of the extended afterglow bolometric luminosity over single EQTSs, in order to determine the radius from which it comes the most of the observed emission at any value of the arrival time. The computation will be separately performed over different selected EQTSs encompassing all the extended afterglow regimes, from the prompt emission all the way to the latest phases. The temporal evolution of the luminosity distribution over the EQTSs will be then presented, together with the corresponding temporal evolution of the EQTS apparent size in the sky.

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